Fiberglass, as a component for products, can be classified by density. Low density fiberglass is generally between two and four pounds per cubic foot. Medium density fiberglass is generally between four and seven pounds per cubic foot. Fiberglass is considered high density if it is above seven pounds per cubic foot. While there is little difference from an acoustical standpoint between one inch thick fiberglass having a density of three pounds per cubic foot or seven pounds per cubic foot, the higher density fiberglass costs more than twice that of the low density material.
Even though the higher density material costs more, the industry has gravitated in that direction. The industry has seen that it is necessary to harden or stiffen the edges of the panel. This has typically been done chemically by applying a liquid resin containing a hardening agent to the edge. When, for example, a polythermal setting resin is applied to the fiberglass core, it wicks into the core to a depth consistent with the amount applied. The resin then hardens binding the edge fibers of the core into a solid mass. The depth of the hardened mass is normally 1/8-1/4.
This technique works better with higher density fiberglass because the air spaces between the fibers are smaller and more fibers are present to hold the liquid resin as it hardens. When this technique is used with low density fiberglass, the edges do not become stiff. Instead, they remain spongy and pliable. Even when used with high density fiberglass, this process is inefficient.
First, the chemical reaction which causes resin to set up normally will take anywhere from one to six hours depending on the temperature, humidity or the amount of catalyst that is mixed with the resin. Second, the panel edges need to be separated while the resin agent hardens otherwise the panel edges will bond together. Third, during application and curing, the resin chemicals typically used emit odors that are difficult to control. As a consequence some manufacturers are switching to water based polymer hardening compounds. While such hardening agents are less costly and do not emit odors, they take at least 24 hours to dry into a hardened state and also require the panels to be separated during the drying period. Fourth, when used on long, narrow panels, the hardening of the resin often causes the panel to bow or warp along its length.
Finally, panels constructed using impregnated techniques cannot be easily and acceptably modified in the field. Panels typically need to be trimmed to fit within a room. Panels also need to be trimmed or cut to permit installation around electrical fixtures and outlets, HVAC vents, plumbing fixtures and sprinkler heads. Also, treating cut edges in the field with a resin stiffener is totally impractical.